The present invention relates in general to communication systems and is particularly directed to a reduced complexity adaptive finite impulse response (FIR) filter for a direct spread spectrum radio, that compensates for the inability of a fixed equalization filter to accommodate changes in the characteristics of the radio due to temperature, aging or other effects.
In order to be competitive, wireless telecommunication service providers who offer data transmission services to business and consumer customers by way of direct spread-spectrum systems have found it necessary to employ low cost (reduced complexity) radio circuit designs. (For a non-limiting example of literature detailing direct sequence spread-spectrum systems, attention may be directed to the text entitled: xe2x80x9cCoherent Spread Spectrum Systems,xe2x80x9d by J. K. Holmes, John Wiley and Sons, 1982, and the text entitled: xe2x80x9cSpread Spectrum Systems with Commercial Applications (Third Edition),xe2x80x9d by R. C. Dickson, John Wiley and Sons, 1994.)
The performance of these radios is influenced by two major sources of signal degradationxe2x80x94random noise, and internal distortion. The first sourcexe2x80x94random noisexe2x80x94is injected into the radio channel, and ultimately limits the performance and range of the radio. The secondxe2x80x94signal distortionxe2x80x94results from filtering and modulation artifacts inherent in the radio.
In order to compensate for these signal degradation sources, low cost direct spreading radio circuits may contain equalization filters, whose characteristics are fixed by design, or tuned to match each radio at the time of production. Even though fixed equalization filters reduce hardware complexity and cost, over time they limit the performance of the radio, as they cannot dynamically track changes in operational characteristics of the radio due to temperature, aging or other effects.
In accordance with the present invention, the above described performance limitations of conventional commercial grade, direct spread-spectrum radios are effectively obviated by inserting a reduced complexity, stepwise or iteratively adaptive FIR filter in the received signal processing path of the radio just upstream of the despreading function. As will be described, in a non-limiting, but preferred embodiment, this adaptive filter may be implemented as a relatively small numbered (e.g., four) tap FIR filter, having its cursor tap (center tap) fixed at a (maximum) value of 2bxe2x88x921xe2x88x921, where b is the number of bits per tap weighting coefficient (e.g., eight).
The remaining tap values of the filter are individually adaptively adjusted, as necessary, by the radio""s control processor, which monitors the error or offset in the data values produced by a data signal analyzer (e.g., bit slicer). In particular, the control processor executes a tap adjustment routine in which it iteratively increments or decrements each variable tap value of the FIR filter to an xe2x80x98optimizedxe2x80x99 value that effectively minimizes the total (I and Q) power in the error in the data decisions performed by data signal analyzer.